This invention relates generally to machining or drilling the surface of a workpiece and more particularly concerns drilling multiple holes in a workpiece to a uniform depth and in the case of a curved workpiece, drilling holes that have axes that are normal or oriented at a predetermined angle to the surface of the workpiece.
In making a molded plastic article such as a golf ball, it is necessary to machine a cavity into a mold into which melted plastic material can be injected. The cavity of the mold has a machined or engraved surface pattern that produces the requisite surface pattern on the molded plastic article. In the case of a golf ball, the surface of the mold cavity has bumps that produce the characteristic dimples on the surface of the golf ball.
It is well known in the art that the internal surface for a mold, can be machined by using an electric discharge machine process. A hemispherical shaped electrode having dimples in its surface is inserted into a round mold cavity, and while applying electric current, the electrode is orbited and reciprocated to produce matching bumps on the surface of the mold cavity. Later when the mold is used for molding a golf ball, matching dimples will be formed on the molded golf ball.
In order to manufacture concave molds having an internal reoccurring pattern such as the dimples on a golf ball, it is necessary to produce that same reoccurring pattern of holes on the surface of the hemispherically shaped electrode. Particularly, it is important that the holes provided in the electrode be of a uniform depth and that the axes of those holes pass through the geometric center of the hemispherically shaped electrode which conforms to the hemispherically shape of the resulting molded golf ball.
When boring holes of a uniform depth on a hemispherical surface, such as an electrode used in the electric discharge machining of a mold cavity for a golf ball, it is necessary to orient the hemispherical electrode with respect to the axis of a boring tool (the z-axis) so that the axis of the tool is aligned with the geometric center of the hemispherical electrode. Consequently, not only does the hemispherical electrode have to move in the horizontal plane of the bed of the machine tool (defined by the x-axis and the y-axis), but also the hemispherical electrode must be pivoted about an axis (b) which lies parallel to the horizontal plane and must rotate about the axis (a) of the hemisphere.
Particularly, with regard to the pivot axis (b), it is difficult to assure that hemispherical surface of the electrode will be accurately oriented when pivoted about the axis (b) because the hemispherical surface extends out from the b-axis by the length of the stem of the electrode. Any tolerance in the pivot angle around the b-axis will result in substantial deviation along the z-axis at the hemispherical surface of the electrode. Consequently, a numerically controlled machine tool cannot accurate drill multiple holes to a uniform depth because of the inability to know, within acceptable tolerance, exactly where the surface of the hemisphere is along the z-axis.
Moreover, because of the inaccurate pivoting of the hemispherical surface with respect to the b-axis, the center of the hemisphere may be moved along the x-axis to a position that is offset from the point where the tool (the z-axis) projects into the horizontal plane defined by the x-axis and the y-axis. That offset of the z-axis will result in the axis of the hole not passing through the center of the hemisphere and therefore not being perpendicular or normal to the hemispherical surface of the electrode.
It is therefore an object of the present invention to provide a method for machining a surface of a workpiece with a tool to provide multiple cuts having uniform depths.
It is likewise an object of the present invention to provide a method for machining a surface of a workpiece with a tool having an axis so that the axis of the tool engages the surface of the workpiece at a predetermined angle.
It is also an object of the present invention to provide a method for machining a surface of a workpiece in which the exact position of the surface of the workpiece is sensed for each cut made in the workpiece, and the exact positions of the workpiece with respect to the expected positions of the workpiece are displayed to insure that each subsequent cut was made to the same uniform depth as the previous cuts in the workpiece.
It is also an object of the present invention to monitor any deviation in the expected position of the workpiece and the exact position of the workpiece to determine that the axis of the tool remains at a predetermined angle to the surface of the workpiece.
The foregoing objectives are accomplished by using a numerical controlled machine tool having five axes of orientation to make multiple cuts in the curved surface of a workpiece, particularly the hemispherical surface of an electrode to be used to electric discharge machine the cavity of a mold for a golf ball. In order to assure uniform depth cuts which are made at a predetermined angle to the surface, the workpiece is insulated from the machine tool, and the workpiece and the tool of the machine tool are connected to an electric circuit which senses the electrical contact between the workpiece and the tool and thereby records the sensed or exact position of the surface of the workpiece. Once the surface of the workpiece has been located by the electric sensing, the machine tool is advanced a predetermined incremental distance to assure a predetermined depth of cut.
The deviation between the sensed or exact position of the surface of the workpiece and the anticipated or expected position of the workpiece based on the orientation of the axes on the machine tool is used to monitor the position of the workpiece to assure that the surface of the workpiece is accurately sensed during subsequent machining steps and to calculate an angle between the surface and the axis of the tool to assure that the axis of the tool is oriented at the predetermined angle to the surface of the workpiece.
In order to assure accurate sensing of the position of the workpiece, the workpiece is sprayed with a nonconductive enamel layer prior to machining so that a false electrical contact is not created by conductive debris which may accumulate on the surface of the workpiece during the machining operation. Likewise, a lubricating, electrically nonconductant coolant is used during the actual machining of the workpiece to assure that a film of liquid on the workpiece does not provide a erroneous path of conduction between the tool and the workpiece thereby providing false sensing of the position of the surface of the workpiece. In addition, we have found that a liquid lubricant coolant which contains a silicone compound minimizes tearing of the copper material of the workpiece thereby providing a more uniform bore and less large particles of debris which may become engaged between the tool and the workpiece thereby providing false sensing of the position of the surface of the workpiece.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings.